The present invention relates to the field of dropframe vehicles, such as semi-trailers or other vehicles in which a wide and low cargo floor is desired. More particularly, the invention relates to a four-wheel drive system and four-wheel steering system for such vehicles.
Virtually every commodity on the market today is, at one time or another, transported by truck. Often the truck is a tractor-trailer rig or in some cases a delivery van or pick-up. It is axiomatic that the larger the cargo space for a fully loaded van or trailer, the lower the per-mile transportation cost for the transported commodity.
Various governmental regulations dictate the maximum dimensions of highway vehicles. What is required, then, is a vehicle that falls within the regulated dimensions while maximizing the total cargo carrying volume of the vehicle. One approach has been to use a so-called "dropframe" for the cargo-carrying body. In this construction, the floor of the cargo compartment is dropped close to the road surface between the suspension apparatus of the trailer or van.
In our prior patent, U.S. Pat. No. 4,934,733, assigned to DBX Corporation, we disclosed a wheel suspension system which when used with a trailer, for instance, permits the floor of the trailer body to be lowered closer to the roadway surface than with the prior suspensions, and which permits the floor to be substantially flat throughout the length of the cargo compartment. This suspension system has been successfully used in a variety of vehicles, including semi-trailer rigs, delivery vans and pick-ups. One limitation of this trailer suspension system is that it has been limited to use with conventional front-wheel drive and steering vehicles.
There is a need in the transportation and shipping industry for a four-wheel drive system and a four-wheel steering system that can be used with the dropframe type trailer suspension apparatus, such as the apparatus disclosed in the '733 patent. The four-wheel drive and steering system must provide adequate clearance for the floor of the dropframe storage compartment, while maintaining performance characteristics comparable to known four-wheel drive and steering systems.